FIG. 8 shows an example of a conventional air conditioning system. This is a combination system in which an indoor air conditioner unit 3 for processing room air by recirculating the indoor air is combined with a ventilation air conditioner 11 to process outdoor air and admit processed outdoor air into indoors. The system is based on an enthalpy heat exchanger, and undertakes heat exchange processes for both humidity and sensible heat in outdoor air and indoor air. Processing load on the air conditioner generated in the conditioned space is extracted by a heat pump system and discharged to outdoors.
The operation of such a system will be explained with reference to a psychrometric chart shown in FIG. 9. During a cooling period, outdoor air (at a state K) and indoor air (at a state Q) exchange enthalpy and outdoor air reaches a state L and indoor air reaches a state T, and respectively become a supply air to indoor space and an exhaust air to be discharged to outdoors. Enthalpy exchange efficiency for this process is as low in the currently available products as in a range of 60.about.70%, so that an enthalpy difference AH is produced between the supply air (at a state L) and room air (at a state Q). The result is that air having excess humidity (i.e., difference in moisture content, .DELTA.X) is supplied to the room so as to introduce moisture corresponding to 30.about.40% of humidity ratio difference between untreated outdoor air and indoor air. This moisture must be removed by the air conditioning system, by cooling the indoor air to 5.about.10.degree. C., which is lower than its dew point (15.about.16.degree. C.).
Of the air processing loads on the air conditioner based on an enthalpy heat exchanger, the latent heat load required for dehumidification is about 10.about.15% of the total load and the remaining 85.about.90% is a sensible heat load. This sensible heat load can be removed at about 15.about.20.degree. C. without cooling the air temperature to the dew point. However, in the conventional systems, because the admitted outdoor air is mixed with the indoor air and the whole air has to be processed, the latent heat is removed only by cooling the air to well below the dew point, to about 10.degree. C. Therefore, the temperature difference (temperature lift) between the evaporator temperature and the condenser temperature in the air conditioner needs to be set the same as in a case when not using the enthalpy heat exchanger, meaning that although the conditioning load on the air conditioner can be reduced, the temperature lift for pumping up the heat cannot be reduced.
It can be seen in that, in the conventional systems, a large amount of temperature lift is necessary for pumping and discharging the heat, therefore, energy consumption in the heat pump for removing sensible heat has been high and wasteful. Also, the facility becomes cumbersome because of the necessity of providing a drain to discharge condensed moisture.